1. Field of the Invention
The present invention relates to an optical isolator, and particularly to an adjustable optical isolator with advantages of high isolation.
2. Description of Related Art
Optical isolators are among the key devices in optical fiber communications which attract attentions as means for materializing a highly information-oriented society in the future. An optical isolator permits the passage of light in one direction through the isolator, but prevents (or at least substantially attenuates) the passage of light in the opposite direction through the isolator.
To achieve such non-reciprocal operation, an optical isolator usually comprises a Faraday rotator which behaves differently, according to the direction in which the light passes through it. A Faraday rotator comprises a material, typically a crystalline material, which is capable of rotating the plane of polarization of light passing through it in response to an external magnetic field. The direction of rotation of the plane of polarization is dependent on both the direction of the light passing through the Faraday rotator and the applied external magnetic field. The Faraday rotator is usually combined with polarizers or birefringent walk off crystals, in order to form an isolator. The rotation of the plane of polarization provided by the Faraday rotator in one direction allows light to pass through both polarizers, whereas in the opposite direction the plane of polarization is rotated so that light is blocked by the polarizer.
U.S. Pat. No. 6,055,102 discloses such an optical isolator. It includes a pair of optical polarizers with their optical axes set at a mutual relative angle of about 45 degrees, and a Faraday rotator having a Faraday rotation angle of about 45 degrees inserted between the optical polarizers. The two polarizers and the sandwiched Faraday rotator are bonded together. The optical isolator functions to allow a forward light from a laser to pass therethrough, and to shut out a backward or return light coming back to the laser. A permanent magnet is provided for applying a static magnetic field to the Faraday rotator, to magnetically saturate or nearly saturate the Faraday rotator. The magnet defines a slot for accommodating the combination of the polarizers and the Faraday rotator therein.
As disclosed in U.S. Pat. Nos. 5,208,876, 5,317,655 and Re. 35,575, one important factor in the performance of optical isolators is the polarizers employed. The two birefringent polarizers of these patents are wedge-shaped, and arranged for optimum optical performance. The first polarizer has a front surface that receives light in the forward direction slanted at an angle "PHgr" varying from 8 degrees to 15 degrees. Similarly, the second polarizer is slanted by the same angle "PHgr" in a complementary fashion to the first polarizer. The slanted surfaces are parallel to each other. The slant of the polarizers reduces the forward reflectivity or return loss, to approximately xe2x88x9260 to 65 dB. This reduction is important for meeting the demands of present-day fiber optic networks.
Another example of a conventional isolator is shown in FIG. 1. An isolator 1xe2x80x2 comprises a pair of collimating sub-assemblies 19xe2x80x2 and a combination, i.e., the isolator core, sandwiched therebetween. The combination has a magnetic ring 12xe2x80x2, a pair of wedge-shaped polarizers 13xe2x80x2, 15xe2x80x2 and a Faraday rotator 14xe2x80x2 disposed between the two polarizers 13xe2x80x2, 15xe2x80x2. The polarizers 13xe2x80x2, 15xe2x80x2, and the rotator 14xe2x80x2 are all fixed within the magnetic ring 12xe2x80x2. Each collimating sub-assembly 19xe2x80x2 comprises a pigtail element 10xe2x80x2 (17xe2x80x2) holding one end of an optical fiber therein and a lens 11xe2x80x2 (16xe2x80x2) which is commonly a GRIN lens.
The polarizers 13xe2x80x2, 15xe2x80x2 of the isolator are fixed together within the magnetic ring 12xe2x80x2. The optical axes of the polarizers 13xe2x80x2, 15xe2x80x2 are required to be crossed at a certain angle, preferably 45 degrees. Thus unduly high manufacturing specifications and standards are required for achieving good isolation of the isolator 1xe2x80x2.
Another isolator disclosed in U.S. Pat. No. 5,359,689, is shown in FIG. 2. It comprises an attaching member 5 holding an optical fiber 4 therein, a ferrule 51 positioned in the attaching member 5 and accommodating a bared end 41 of the fiber 4, a holder 52 positioned at one end of the attaching member 5 and receiving a slanted end of the ferrule 51 therein, a magnet 54 attached to the holder 52, and a sleeve 53 enclosing the magnet 54 and fixed to the attaching member 5. An isolating combination includes a pair of birefringent elements 42, 44 and a Faraday rotator 43 sandwiched therebetween. The birefringent element 44 is attached in the holder 52 together with the slanted end of ferrule 41.
This isolator is relatively easy to mass-produce. However, it still suffers from much the same design shortcomings as discussed above regarding the isolator 1xe2x80x2. That is, high manufacturing precision is still required thereby limiting any reductions in costs.
An object of the present invention is to provide an adjustable optical isolator having high isolation.
Another object of the present invention is to provide a low-cost optical isolator.
In accordance with one aspect of the present invention, an optical isolator includes a first pigtail assembly, a first collimating lens aligned with the first pigtail assembly, an optical rotator assembly aligned with the first pigtail assembly and the collimating lens, an adjustable assembly aligned with the rotator assembly, and a second pigtail assembly aligned with the adjustable assembly. The rotator assembly includes a first birefringent wedge, an optical rotator, and a magnet for enclosing the first birefringent wedge and the rotator therein. The adjustable assembly comprises a second birefringent wedge, a second collimating lens, and a holder holding at least a portion of the second birefringent wedge and at least a portion of the second collimating lens therein. The first and second pigtail assemblies each have a ferrule and an optical fiber with an end attached within the ferrule.
The rotator is positioned between the first and second birefringent wedges and the position of the adjustable assembly is adjustable relative to the rotator assembly. This achieves precise positional relationships among the rotator and the first and second birefringent wedges.
The first birefringent wedge has a first slanted surface, and the second birefringent wedge has a second slanted surface parallel to the first slanted surface. The first and second slanted surfaces of the first and second birefringent wedges may both confront the rotator, or both face away from the rotator. Alternatively, the rotator may form a pair of opposite surfaces which are slanted and parallel to the first and second slanted surfaces of the first and second birefringent wedges.